Contents Telektronikk - Telenor

- some upward preference for low overload,
e.g. Bd =1.9 %, Bu = 1.1 %
- downward preference for higher overload,
Bd = 35 %, Bu = 56 %.
This is quite in accordance with our
objective.
Circuit reservation may also be used
partly to protect, partly to compensate the
GOS on a final route with overflow from
direct routes. R = 2 will be satisfactory,
giving some protection of the final route.
Several authors have studied this. [2] is
recommended.
In the Norwegian network rerouting is
planned to ensure that the capacity of
both circuit groups is available also for
downward traffic. However, circuit reservation
may be used in addition to
obtain downward preference and reduce
the amount of rerouted calls.
7.3 Protection of normal traffic
load
The objective of dynamic routing is to
utilise the capacity of the network even if
the traffic distribution differs from the
dimensioned. Two different strategies
may be emphasised:
1 The network should be utilised in a
way that gives maximum throughput.
One consequence could be that a large
proportion of a network is affected by
a failure situation.
2 Dynamic routed traffic is only allowed
as long as it does not severely affect
the traffic normally carried on the circuit
group. This may be achieved if
dynamic routing does not influence the
GOS of direct routed traffic more than
a fixed percentage.
Dynamic routing in a mesh network may
result in instability as two link traffic is
competing with direct routed traffic. Circuit
reservation against DAR traffic is
necessary to avoid this. A reservation
level of R = 1/2 SQRT(N), where N is the
size of the circuit group, is recommended
in order to give maximum throughput in
accordance with the first objective above.
Simulations by Norwegian Telecom
Research [3] support this result. Some
calculations using the formulas in
Appendix 1 are set up in Figure 7. GOS
1 % at normal traffic is assumed. Also
the overload traffic is Poisson traffic in
the example. The results show some variation
of the blocking of the normal traffic,
largest for N = 60 (from 4 to 11 %).
The recommended dimensioning
principle in Telenor, standard
dimensioning (see chapter 9), is
more robust towards overload. As
shown in Figure 8 this makes it
possible to put emphasis on the second
objective above by selecting
the same reservation parameter,
R, for all circuit groups independent
of circuit group size.
With our constraints R = 7 limits
the blocking of normal traffic to
5 %, similarly R = 10 limits the
blocking to about 3 %.
8 Dynamic Alternative
Routing
Dynamic Alternative Routing
(DAR) is a method that will be used
in the top mesh network between
TEs as an addition to the direct
routing. DAR may be explained by
the following example in Figure 9.
A call has entered the mesh network
in trunk exchange A. Its destination
is end office X. From A to X
the call is routed as follows:
- 1st choice is always the load
sharing group AB/AC. Both circuit
groups will be tried before
the call eventually overflows to a
3rd alternative, the DAR-alternative.
- The DAR-alternative is determined
by a pointer in a circular list
of alternatives (DAR-list) exclusive
for this traffic destination
(TE-region BC). In the example
exchange E is the current alternative.
This alternative is selected if
the circuit groups AE
and either EB or EC
have free capacity
for DAR traffic
(according to the cir- DAR-list
cuit reservation
level).
- If the call is blocked
on AE, the first leg
of the DAR-alternative,
the call will be
given one more alter- A
native. The DARpointer
will be
stepped to F and the
call is offered the circuit
group AF.
- If the call is successfully
set up to B or
C, there is no need to
Blocking Normal Traffic (%)
12
10
8
6
4
2
R= 4, N= 60
R= 7, N= 180
R=10, N= 390
0
120 150
200 400
Total Traffic Offered (% of Normal Traffic Load)
Figure 7 Protection of normal traffic (GOS 1 %) with reservation
level R = 1/2 SQRT(N)
Blocking Normal Traffic (%)
12
10
8
6
4
2
N = 60
N = 180
N = 390
R=4
R=7
R=10
0
120 150
200 400
Total Traffic Offered (% of Normal Traffic Load)
Figure 8 Protection of normal traffic (standard dimensioning)
with fixed reservation level
DAR-pointer
D E F K
Figure 9 Example of Dynamic Alternative Routing (DAR) in the top level
mesh network
B
C
X
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